In the LTE (Long Term Evolution) scheme defined in the 3GPP, the “AMC (Adaptive Modulation and Coding) control” is applied in which a radio base station Enb controls a modulation scheme or a channel coding rate of the uplink shared channel (PUSCH: Physical Uplink Shared Channel), based on a reception quality (e.g., received power, a signal-to-interference ratio (SIR), a signal-to-interference-plus-noise ratio (SINR), and a path loss) in a measured uplink.
In this case, in a certain modulation scheme, TBS (Transport Block Size), which is the number of bits that can be transmitted by a predetermined frequency resource per unit time (e.g., a radio resource in resource block unit), is determined according to a channel coding rate.
Therefore, in the AMC control of the uplink of the LTE scheme, it is configured to control the modulation scheme and TBS, based on the reception quality of the uplink such as the reception SIR.
In this case, as illustrated in FIG. 5(a), in the HSDPA (High Speed Downlink Packet Access) scheme defined in the 3GPP, a code resource (radio resource) for the downlink shared data channel (HS-DSCH: High Speed Downlink Shared Channel) is configured to be assigned to each mobile station MS by each time slot, and an assignment unit of the radio resource is constant.
On the other hand, as illustrated in FIG. 5(b), in an uplink of the LTE (Long Term Evolution) scheme defined in the 3GPP, a radio resource for PUSCH is configured to be assigned to each mobile station MS by each time slot (sub-frame).
The radio resource in the uplink of the LTE scheme is configured to be assigned to the uplink shared channel in resource block unit in which a system bandwidth is divided by each predetermined bandwidth.
Specifically, the resource block is a minimum assignment unit of a radio resource for an uplink shared channel defined in a two-dimensional plane represented by a frequency direction and a time direction, and is configured by seven SC (Single Carrier)-FDMA symbols in the time direction and twelve sub-carriers in the frequency direction.
It is noted that an element configuring the resource block is referred to as “resource element”, and each resource block is configured by 12×7 resource elements (=SC-FDMA symbols).
In the uplink of the LTE scheme, the mobile station UE transmits control information (transmission acknowledgement information for the downlink shared channel, reception quality information (Channel Quality Indicator), a Scheduling Request, etc.) through PUCCH (Physical Uplink Control Channel) at a timing at which PUSCH is not transmitted.
However, in the LTE scheme, when uplink data and the control information are transmitted through PUSCH at the same timing, the uplink data channel and the control information are configured to be multiplexed on and mapped to the resource block assigned to the mobile station UE. And, the number of resource elements to which the control information is mapped varies depending on an amount of the control information to be transmitted. As a result, the number of bits after channel coding in the uplink shared channel is changed and a channel coding rate in the uplink shared channel is differed even when the same modulation scheme and TBS are selected. This results in a problem that a desired transmission quality (e.g., BLER) cannot be realized.
Further, in the uplink of the LTE scheme, in order to maintain synchronization of the uplink and to measure a radio channel state in the uplink at the radio base station, the mobile station UE transmits a reference signal that is referred to as “Sounding Reference Signal (SRS)”. The SRS is multiplexed on an SC-FDMA symbol at the tail end of a specific sub-frame designated by the radio base station.
Therefore, the number of resource elements that can be used in the uplink shared channel varies depending on whether or not the SRS is multiplexed including SRS at another mobile station, and a channel coding rate in the uplink shared channel is differed even when the same modulation method and TBS are selected. As a result, there is a problem that the desired transmission quality (e.g., BLER) cannot be realized.
Therefore, the present invention is intended to overcome the above-described problem. An object of the present invention is to provide a radio base station capable of realizing a desired transmission quality (e.g., BLER) in the uplink data channel even when the number of resource elements to which control information is mapped is not constant and irrespective of whether SRS is multiplexed in the same sub-frame, and to provide a communication control method herefore.